In the field of lubricating oils, additives have been added to lubricating base oils such as highly refined mineral oils to improve the properties such as the viscosity-temperature characteristic or heat and oxidation stability of the lubricating oils (for example, see Patent documents 1-8).
For example, lubricating oils used in internal combustion engines such as automobile engines require heat and oxidation stability that allows them to withstand harsh conditions for prolonged periods. In order to ensure heat and oxidation stability of conventional lubricating oil for internal combustion engines, it is common to use highly refined base oils such as hycracked mineral oils or high performance base oils such as synthetic oils, with addition of peroxide-decomposable sulfur-containing compounds such as zinc dithiophosphate (ZDTP), molybdenum dithiocarbaminate (MoDTC), or ashless antioxidants such as phenol-based or amine-based antioxidants to the base oils (for example, see Patent documents 1 and 4-6).
With the recent emphasis on environmental issues such as reduction in carbon dioxide gas emissions, reduced energy consumption (increased fuel efficiency) of automobiles, construction equipment, agricultural machinery and the like has become a matter of urgency, and it is highly desirable for drive transmission devices such as gearboxes and final reduction gear boxes to help contribute to reduced energy consumption. Increased fuel efficiency for power train devices can be achieved by methods that lower the viscosity of the lubricating oil to reduce stirring resistance and friction resistance against sliding surfaces. For example, gearboxes used as automobile automatic transmissions or continously variable transmissions comprise a torque converter, wet clutch, gear bearing mechanism, oil pump, overpressure control mechanism and the like, while manual transmissions and final reduction gear boxs include a gear bearing mechanism, and by reducing the viscosity of the lubricating oils used therein to lower stirring resistance and friction resistance, it is possible to improve power transmission efficiency and achieve fuel savings.
However, reducing the viscosity of the lubricating oil also results in lower lubricity (antiwear property, anti-seizing properties, fatigue life, etc.), which is disadvantageous for gearboxes and the like. Also, phosphorus-based extreme-pressure agents that are added to guarantee antiwear property and the like for lubricating oils with reduced viscosity can significantly shorten the fatigue life. Sulfur-containing extreme-pressure agents are effective for improving fatigue life, but it is generally known that the effect of the lubricating base oil viscosity in low viscosity lubricating base oils is greater than that of the additives.
One strategy for ensuring lubricity when lowering the viscosity of lubricating oils for increased fuel efficiency has been to optimize the combinations of phosphorus-based extreme-pressure agents and sulfur-containing extreme-pressure agents added to lubricating base oils (for example, see Patent documents 7 and 8).    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 4-36391    [Patent document 2] Japanese Unexamined Patent Publication HEI No. 4-68082    [Patent document 3] Japanese Unexamined Patent Publication HEI No. 4-120193    [Patent document 4] Japanese Unexamined Patent Publication SHO No. 63-223094    [Patent document 5] Japanese Unexamined Patent Publication HEI No. 8-302378    [Patent document 6] Japanese Unexamined Patent Publication HEI No. 9-003463    [Patent document 7] Japanese Unexamined Patent Publication No. 2004-262979    [Patent document 8] Japanese Unexamined Patent Publication No. 2004-262980